Throughout the several drawings forming a part of this disclosure, like numerals represent the same or functionally similar elements. FIGS. 1 and 2 are schematics of known CFB boiler systems used in the production of steam for industrial process requirements and/or electric power generation. Referring to FIG. 1, fuel and sorbent are supplied to a bottom portion of a furnace 1 contained within enclosure walls 2, which are normally fluid cooled tubes. Air 3 for combustion and fluidization is provided to a windbox 4 and enters the furnace 1 through apertures in a distribution plate 5. Flue gas and entrained particles/solids 6 flow upwardly through the furnace 1, releasing heat to the enclosure walls 2. In most designs, additional air is supplied to the furnace 1 via overfire air supply ducts 7.
The system of FIG. 1 provides two stages of particle separation: in-furnace impact-type particle separators or U-beams 13 and external impact-type particle separators or U-beams 14. Since the particular designs of such U-beams configurations and their functions have been previously disclosed (see, for example, U.S. Pat. Nos. 4,992,085 and 4,891,052 to Belin, et al. and U.S. Pat. No. 5,343,830 to Alexander et al., all assigned to The Babcock & Wilcox Company), and they will not be discussed in further detail. Suffice it to say that the in-furnace U-beams return their collected particles directly into the furnace 1, while the external U-beams return their collected particles into the furnace via the particle storage hopper 11 and L-valve 12, collectively referred to as a particle return system 15. An aeration port 16 supplies air for controlling the flow rate of solids or particles through the L-valve 12.
The flue gas and solids 6 pass into a convection pass 17 which contains convection heating surface 18. The convection heating surface 18 can be evaporating, economizer, or superheater as required.
Although not shown in connection with the FIG. 1 system, an air heater would also be provided downstream of the convection pass 17 to extract further heat from the flue gas and solids 6. A multiclone dust collector (also not shown) would also be supplied to recycle solids back to a lower portion of the furnace enclosure.
In CFB reactors, the reacting and non-reacting solids are entrained within the reactor enclosure by the upward gas flow which carries solids to the exit at the upper portion of the reactor where the solids are separated by the internal and/or external particle separators. The collected solids are returned to the bottom of the reactor commonly by means of internal or external conduits. In the system of FIG. 1, the L-valve 12 is a pressure seal device that is needed as a part of the return conduit due to the high pressure differential between the bottom of the reactor and the particle separator outlet. The primary separator at the reactor exit collects most of the circulating solids (typically from 95% to 99.5%). In many cases an additional (secondary) particle separator and associated recycle means are used to minimize the loss of circulating solids due to inefficiency of the primary separator.
The internal impact-type particle separators are comprised of a plurality of concave impingement or impact members supported within the furnace enclosure and extending vertically in at least two rows across the furnace exit opening. Collected particles fall unobstructed and unchannelled underneath the collecting members along the enclosure wall. This separator has proven effective in increasing the average density in a CFB combustor without increasing the flow of externally collected and recycled solids, while still providing simplicity of the separator structural arrangement, absence of clogging, and uniformity of the gas flow at the furnace exit. The latter effect is important to prevent local erosion of the enclosure walls and in-furnace heating surfaces like wingwalls caused by impingement of a high velocity gas-solids stream.
In this known embodiment, the internal impact-type particle separator, comprised of two rows of impingement members, is typically used in combination with a downstream external impact-type particle separator from which collected solids are returned to the furnace by an external conduit. The external impact-type particle separator and associated particle return means, e.g., the particle storage hopper and L-valve of FIG. 1, are needed since the efficiency of the internal impact-type particle separator, comprised typically of two rows of impingement members, is not sufficient to prevent excessive solids carryover to the downstream convection gas pass which may cause erosion of the convection surfaces and an increase of the required capacity of the secondary particle collection/recycle equipment.
U.S. Pat. No. 5,343,830 to Alexander et al., also assigned to The Babcock & Wilcox Company, discloses an entirely new type of CFB reactor or combustor which provides for internal return of all primary collected solids to a bottom portion of the reactor or combustor for subsequent recirculation without external and internal recycle conduits. FIG. 2 is a schematic representation of such an internal recycle, circulating fluidized bed (IR-CFB) boiler, generally designated 30.
In FIG. 2, the front of the CFB boiler 30 or reactor enclosure 32 is defined as the left hand side of FIG. 2, the rear of the CFB boiler 30 or reactor enclosure 32 is defined as the right hand side of FIG. 2, and the width of the CFB boiler 30 or reactor enclosure 32 is perpendicular to the plane of the paper on which FIG. 2 is drawn.
The CFB boiler 30 has a furnace or reactor enclosure 32, typically rectangular in cross-section, and partially defined by fluid cooled enclosure walls 34. The enclosure walls are typically tubes separated from one another by a steel membrane to achieve a gas-tight enclosure 32. The reactor enclosure 32 is further defined by having a lower portion 36, an upper portion 38, and an exit opening 40 located at an outlet of the upper portion 38. Fuel, such as coal, and sorbent, such as limestone, indicated at 42, are provided to the lower portion 36 in a regulated and metered fashion by any conventional means known to those skilled in the art. By way of example and not limitation, typical equipment that would be used include gravimetric feeders, rotary valves and injection screws. Primary air, indicated at 44, is provided to the lower portion 36 via windbox 46 and distribution plate 48 connected thereto. Bed drain 50 removes ash and other debris from the lower portion 36 as required, and overfire air supply ports 52,54 supply the balance of the air needed for combustion.
A flue gas/solids mixture 56 produced by the CFB combustion process flows upwardly through the reactor enclosure 32 from the lower portion 36 to the upper portion 38, transferring a portion of the heat contained therein to the fluid cooled enclosure walls 34. A primary, impact-type particle separator 58 is located within the upper portion 38 of the reactor enclosure 32, and comprises four to six rows of concave impingement members 60, arranged in two groups--an upstream group 62 having two rows and a downstream group 64 having two to four rows, preferably three rows. Members 60 are supported from roof 66 of the reactor enclosure 32 and are non-planar; they may be U-shaped, E-shaped, W-shaped or any other shape as long as they have a concave surface. The first two rows of members 60 are staggered with respect to each other such that the flue gas/solids 56 passes through them enabling the entrained solid particles to strike this concave surface; the second two to four rows of members 60 are likewise staggered with respect to each other. The upstream group 62 of impingement members 60 will collect particles entrained in the gas and cause them to free fall internally and directly down towards the bottom portion 36 of the reactor enclosure 32, against the crossing flow of flue gas/solids 56.
Impingement members 60 are positioned within the upper portion 38 of the reactor enclosure 32 fully across and just upstream of exit opening 40. Besides covering exit opening 40, each impingement member 60 in downstream group 64 also extends beyond a lower elevation or workpoint 68 of exit opening 40 by approximately one foot. In the preferred embodiment, however, and in contrast to the impingement members 60 of upstream group 62, the lower ends of the impingement members 60 in downstream group 64 extend into a cavity means 70, located entirely within the reactor enclosure 32, for receiving collected particles as they fall from the downstream group 64.
The particles collected by downstream group 64 must also be returned to the bottom portion 36 of the reactor enclosure 32. Returning means 72 are thus provided, connected to the cavity means 70 and also located entirely within the reactor enclosure 32. Returning means 72 returns particles from the cavity means 70 directly and internally into the reactor enclosure 32 so that they fall unobstructed and unchanneled down along the enclosure walls 34 to the bottom portion 36 of the reactor enclosure 32 for subsequent recirculation. In this embodiment, the cavity means 70 functions as more of a temporary transfer mechanism, rather than as a place where particles are stored for any significant period of time. By causing the particles to fall along the enclosure walls 34, the possibility of reentrainment in the upwardly flowing gas/solids 56 passing through the reactor enclosure 32 is minimized.
Connected to the exit opening 40 of the reactor enclosure 32 is convection pass 74. After passing first across upstream group 62 and then across downstream group 64, the flue gas/solids 56 (whose solids content has been markedly reduced, but which still contains some fine particles not removed by the primary, impact-type particle separator 58) exit the reactor enclosure 32 and enters convection pass 74. Located within the convection pass 74 is the heat transfer surface 75 required by the particular design of CFB boiler 30. Various arrangements are possible, and the reader is referred to U.S. Pat. No. 5,343,830 for further details. Different types of heat transfer surface 75, such as evaporating surface, economizer, superheater, or air heater and the like could also be located within the convection pass 74, limited only by the process steam or utility power generation requirements and the thermodynamic limitations known to those skilled in the art.
After passing across all or a part of the heating surface in the convection pass 74, the flue gas/solids 56 is passed through a secondary particle separation device 78, typically a multiclone dust collector, for removal of most of the particles 80 remaining in the gas. These particles 80 are also returned to the lower portion 36 of the reactor enclosure 32 by means of a secondary particle return system 82. The cleaned flue gas is then passed through an air heater 84 used to preheat the incoming air for combustion provided by a fan 86. Cooled and cleaned flue gas 88 is then passed to a final particle collector 89, such as an electrostatic precipitator or baghouse, through an induced draft fan 90 and stack 91.
Known IR-CFBs of the type disclosed in Alexander et al. have a single furnace exit opening 40 associated with the arrangement of impact-type primary particle separators. In these furnaces, the furnace dimension perpendicular to the plane of the exit opening 40, i.e., the furnace depth D, is limited in size to a value equal to approximately one-half of the maximum height of the primary impact-type particle separators or U-beams. As indicated above, the maximum height of the U-beams is determined by consideration of the maximum allowable stresses in the U-beams and the particle collection efficiency, which tends to decrease as the U-beams length increases. As a practical limit, the furnace depth is thus limited to a value of approximately 15 feet. For IR-CFB furnaces of large capacity, (100-200 MW.sub.e and larger) this furnace depth limitation results in a prohibitively large furnace aspect ratio (defined as the ratio of the furnace width divided by the furnace depth).
Additionally, in such known IR-CFB designs, the fuel is typically fed by multiple feeders through the furnace front wall. Limestone or sorbent is fed together with fuel or through separate injection points in the front wall and sometimes the rear wall. Solids are also recirculated from the secondary particle separator through the rear wall, and to improve mixing in the lower furnace and to enhance solids entrainment at partial loads, the furnace is generally tapered in its lower part. Secondary air nozzles are also installed at the front and rear walls in this tapered portion of the furnace.
It is thus apparent that an improved IR-CFB reactor or combustor suitable for larger steam generator capacities could be obtained if the furnace depth limitation could be overcome.